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A258494 Number of words of length 2n such that all letters of the septenary alphabet occur at least once and are introduced in ascending order and which can be built by repeatedly inserting doublets into the initially empty word. 2
429, 32032, 1447992, 51647700, 1605746373, 45730269662, 1227377689395, 31600247019120, 789604855530855, 19302666179660304, 464277874912868104, 11032638071392824348, 259801742258903758053, 6076538940891732415102, 141407795779400734204647 (list; graph; refs; listen; history; text; internal format)
OFFSET

7,1

LINKS

Alois P. Heinz, Table of n, a(n) for n = 7..700

FORMULA

a(n) ~ 24^n / (3000*sqrt(Pi)*n^(3/2)). - Vaclav Kotesovec, Jun 01 2015

MAPLE

A:= proc(n, k) option remember; `if`(n=0, 1, k/n*

      add(binomial(2*n, j)*(n-j)*(k-1)^j, j=0..n-1))

    end:

T:= (n, k)-> add((-1)^i*A(n, k-i)/(i!*(k-i)!), i=0..k):

a:= n-> T(n, 7):

seq(a(n), n=7..25);

CROSSREFS

Column k=7 of A256117.

Sequence in context: A064304 A264180 A270412 * A258395 A215547 A181195

Adjacent sequences:  A258491 A258492 A258493 * A258495 A258496 A258497

KEYWORD

nonn

AUTHOR

Alois P. Heinz, May 31 2015

STATUS

approved

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Last modified May 8 06:39 EDT 2021. Contains 343653 sequences. (Running on oeis4.)